Logic devices having more than two logic levels are well known in the art. Such devices having three logic levels are known as trinary logic devices.
One such trinary logic device is described in Braddock U.S. Pat. No. 3,662,193. Therein, a clocked trinary flip flop latch is described in terms of a plurality of AND and OR gates. In operation, the latch provides one of three output level signals during a clock pulse, the signal maintains its level thereafter until injection of an input condition change by the next following clock pulse. The device is incapable of communicating with external devices which may generate optical output devices or which may only respond to optical (or other frequency range) signals.
A number of different trinary logic circuits are disclosed in Turecki U.S. Pat. No. 3,508,033, including a trinary counter and a three state latch circuit. These circuits, too, suffer from the disadvantage of being unable to communicate with or perform logic operations on externally generated signals represented by specific frequency components.
Wolff U.S. Pat. No. 3,090,923 teaches logic circuits which incorporate modulators or mixers for performing logic operations on signals of various frequencies. One frequency represents a logic one level, and another frequency represents a logic zero level, for example. Thus, frequencies of a number of signals in the circuit are changed as a result of binary logic operations performed on combinations of the signals.
The circuitry used for performing the logic operations on the frequency signals is complex, however, and, as with the other prior art circuits, requires the provision of substantial wiring to implement and to communicate with input and output devices.
Although the use of optoelectronic circuits in logic devices has been suggested, as provided at Yhap U.S. Pat. No. 2,952,792, for example, such devices fail to utilize optical frequency signals for inputting and outputting logic signals and for communicating with external devices. In the '792 patent, optoelectronic devices are used to form a basic, or universal, logic block using photoconductors as switching elements and electroluminescent lamps facing related photoconductors as switch operators. Thus, although logic operations are performed on optical signals, the circuit is complex and requires an individualized custom design effort to produce.
There is thus a need in the prior art for logic devices utilizing simple, standard circuits capable of performing multi-level logic operations, such as trinary logic operations, and which are capable of communicating with external devices using optically encoded signals. More specifically, there is a need for logic devices capable of performing logic operations on optical (or other frequency range) signals and which do not require specialized optical frequency range operating devices.